Postflight orthostatic intolerance (POI) is a condition describing the dizziness and fainting that astronauts experience when they are subjected to gravity after becoming acclimatized to microgravity. POI afflicts a significant fraction of male and nearly all female astronauts upon return to Earth from orbital Space Shuttle missions. No instances of POI on the moon were reported from the limited number of male astronauts who participated in lunar landings (1/6 G), but possible symptoms of POI on the moon and Mars (3/8 G) are potentially catastrophic due to the more hazardous conditions and lack of medical facilities. In addition, the long duration flights necessary to reach Mars may elicit adaptations that increase the risk of POI. A number of countermeasures have been proposed and some have been tested during return to Earth, but testing on the moon and Mars is obviously problematic. Of these countermeasures, artificial gravity (centrifuge training) has been identified as a treatment with high likelihood of success. Therefore, computer modeling was undertaken to compare the orthostatic response of male and female astronauts during stand tests in constant gravity, for which the gravitational body force is constant, and in centrifuge conditions, for which the centrifugal body force increases with distance from the centrifuge axis.

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